Such cells are used in many applications, and in particular in fluidic oscillator flow meters.
A flow meter of that type is symmetrical about a longitudinal plane, and the flow of fluid therein is transformed into a fluid jet which oscillates in a "oscillation" chamber transversely relative to said plane at an oscillation frequency that is proportional to the flow rate of the fluid.
The variations in differential pressure which appear between two positions taken up in alternation by the fluid jet symmetrically about the above-defined longitudinal plane are converted by a cell into an electric signal representative of their frequency of oscillation.
Associated electronics transform the electric signal into a squarewave that can be used to determine the flow rate of fluid in the oscillator, and also the volume that has passed therethrough.
It is thus possible to provide a channel connecting two pressure takeoff points corresponding respectively to two symmetrical positions of the fluid jet, and to place a converter cell in the channel between the two pressure takeoff points corresponding to the two extreme symmetrical positions of the fluid jet in order to obtain the electric signal representative of the differential pressure.
The converter cell has two chambers filled with an incompressible fluid such as oil and separated by a rigid wall, each of the chambers also being defined by a membrane remote from the wall.
Each of the membranes is in contact with the fluid of the channel and is subjected directly to pressure by said fluid. As a result, both membranes are continuously subjected to the action of different pressures that vary in alternating manner depending on the oscillation of the jet. These pressures are transmitted to the separating wall via the incompressible fluid present in the chambers.
The wall separating the chambers carries a differential pressure sensor which is in communication with each of the chambers so as to be exposed to the pressures obtaining therein.
By way of example, the sensor may be the sensor described in document CH 680 392 which comprises a fixed central element between two deformable elements each of which is subjected to the pressure obtaining in the corresponding chamber.
Two parallel cavities in communication are formed respectively between each deformable element and the central element to enable the sensor to operate. Each couple comprising the central element and one of the deformable elements is provided with respective electrodes disposed facing each other and forming a capacitance which varies in different manner when different pressures are applied to each of the deformable elements.
When the chambers are filled with the incompressible fluid and are then sealed, it is highly likely that the filling and sealing operations are not identical in both chambers, so there is a risk of generating a pressure difference between the chambers.
The pressure difference can be further increased if the volumes of the chambers are not absolutely identical and the membranes do not have exactly the same stiffness.
Because of this "parasitic" pressure difference, the deformable element(s) of the sensor is/are subject to residual deformation giving rise automatically to error in subsequent measurements.
That phenomenon is made worse when the temperature of the medium in which the converter cell is to be found increases at a frequency that is low or zero (a non-repetitive phenomenon) since, under such circumstances, the volumes of the chambers increase differently from one chamber to the other, thereby giving rise to greater residual deformation of the deformable element(s) and thus inducing even greater error before any measurement is performed.
To avoid the above, special care needs to be taken during manufacture of converter cells, thereby making the manufacturing process more complex and increasing the cost thereof.